1. Field of the Invention
The present invention relates to an electronic photographing apparatus which uses a two-component developer comprising a mixture of a toner and a carrier as a magnetic particle.
2. Description of the Related Art
On an electronic photographing apparatus, a photosensitive body is charged and exposure is performed in correspondence with image data to form charge distribution in accordance with an image pattern on the photosensitive body. In a development process, an electronic latent image is developed with toner in accordance with the charge distribution to cause the latent image to appear as a visible toner image. Then the toner image is transferred onto paper and fixed on the paper as an image by way of thermal fixing.
For the development process, there is provided a development system using a two-component developer comprising a mixture of a toner as a resin-coated colored powder particle 10 micrometers in grain diameter and a carrier as a magnetic particle of ferrite, magnetite or iron powder 50 to 150 micrometers in grain diameter.
A development unit uses a developer roll comprising an internal magnet and an external rotary cylinder to convey a developer to a development section as a gap between the photosensitive body and the developer roll. The developer conveyed to the development section is subject to an electric field determined by the relationship between a development bias voltage applied to the developer roll and surface potential distribution determined by the surface charge distribution of the photosensitive body. The toner component in the developer thus adheres to the photosensitive body in accordance with the surface charge distribution.
A method is known which uses a development bias voltage comprising an AC voltage superimposed on a DC voltage (for example, JP-B-63-25350, JP-B-3-02304). The advantage of the method using an AC voltage in the development bias voltage is that the amount of a toner used for development can be increased by superimposing an AC voltage on a DC voltage, compared with a case where the DC voltage alone is used, even in case the DC voltage is low. The advantage of the method is noticeable when it is used for non-contact development where a developer does not come in contact with a photosensitive body.
Further, approaches are known where the ratio of the volume occupied by magnetic particles to the space of a development section is 1.5 to 30 percent and the amount of a developer on the developer roll is 5 to 50 mg/cm2 (for example, JP-B-01534) and setting the electric resistivity of a carrier to be used within a predetermined range (for example, JP-B-7-62779). As a related technology, an approach is known where the frequency of an AC component is set within a range of from 1000 to 3000 Hz (for example, Japanese Patent No. 2646221). The technologies help provide a good-quality image in case an AC voltage is superimposed on a development bias to acquire a development bias voltage.